Method of reducing shrinkage defects in metal castings



Nov. 22, 1955 E. SCHEUER 2,724,160

METHOD OF REDUCING SHRINKAGE DEFECTS IN METAL CASTINGS Filed June 8,1951 AlmWidl I Ill/1,1111

United States Patent METHOD OF REDUCING SHRINKAGE DEFECTS IN METALCASTINGS Ernst Scheuer, Stone, England, as siguor to InternationalAlloys Limited, Buekinghanishire, England Application June 8, 1951,Serial No.'230,636

2 Claims. (01'. 22 214) In connection with the production of metalcastings it is known that gases dissolved in the liquid metal greatlyinfluence the quality of the castings. Generally these gases areconsidered to be objectionable owing to the occurrence of cavities inthe castings which are filled with the gas released from the liquidmetal during its cooling and solidification. However, it has been foundrecently that there are instances Where gas has to be introducedintentionally into the liquid metal in order to make the industrialproduction of useable castings possible without excessive anduneconomical elaboration of the foundry technique. The reason for thisis that it is not always possible, without resorting to complicatedprecautions, to attain in the solidifying casting a temperaturedistribution that will ensure perfect feeding of all parts of thecasting during solidification of the cast metal. The parts that cannotbe completely fed with liquid metal will produce shrinkage cavities andthese shrinkage cavities in certain types of alloys appear in the formof depressions in the surface of the casting, or may even lead to theformation of inter-crystalline cracks.

If the liquid metal contains a sufficient amount of dissolved gas whichis released during solidification of the cast metal, the distributed gascavities produced during such solidification will counteract theformation of shrinkage holes in the places where the solidificationoccurs last,

and will therefore prevent the formation of pronounced local depressionsor inter-crystalline cracks. Accordingly, it has become the practice inindustrial foundries, especially aluminium foundries, to incorporate gasin the liquid metal in those cases Where defects of shrinkage andinter-crystalline cracks cannot be eliminated economically by feedingarrangements. Obviously it is desirable to limit the gas content of theliquid metal to the amount absolutely necessary to achieve theelimination of the shrinkage defects, as too high a gas content wouldunnecessarily weaken the casting by the production of an excessivenumber of gas cavities.

The problem of controlling the gas content of the liquid melt in thefoundry is not simple, because there is no simple means of measuring thedissolved gas in a con venient way for practical purposes. Further, thegas content once established does not remain consistent for any durationof time because the gas dissolved is lost through the surface of themelt into the atmosphere.

In the case of aluminium alloys and magnesium alloys the only gascapable of producing the desired effects is hydrogen; copper alloys canbe gassed by hydrogen or sulphur dioxide or steam; steel can be gassedby carbon monoxide. In order to keep a certain amount of the gas in theliquid melt without loss it would be necessary to maintain anappropriate and uniform concentrationof the gas above the melt. This isnot generally possible in practice as it would mean enclosing the top ofthe melt by a lid. For casting, especially for the production of smallercastings, the top of the melt must be accessible in order to take outmetal. If this is done, air will enter and upset the concentration ofthe gas in contact with the metal, with the result that gas es'capesfrom the liquid metal into the atmosphere.

For practical means of introducing the required gas so far in use is theplunging into the melt of chemical substances that release the gas whenin contact with the molten metal either by heat or by chemical reaction.For example, in the case of molten aluminium, water vapour is bubbledthrough the melt by plunging into the melt green wood or asbestos woolsoaked in water; or a water containing salt is used, or hydrogencontaining substances such as certain oils or ammonium chloride, thehydrogen content being released either by decomposition at thetemperature of the melt, or by the chemical action of the aluminium.These procedures allow a" certain amount of control of the hydrogencontent of the melt immediately after the end of the reaction by usingpredetermined quantities of the reagent, but this control is obviouslyvery rough, as the hydrogen is not all absorbed, but part of it simplybubbles through the melt and the quantity of hydrogen retained is to alarge extent dependent on the method of plunging and the tools used forplunging, and also the shape of the bath. If the hydrogen contentrequired for good casting is comparatively high the loss of hydrogenthrough the surface immediately after the introduction of hydrogen byplung ing is rapid, so that after a period as short as 4 or 5" minutesthe hydrogen content is again down below the required minimum, even if aconsiderable excess of hydrogen is introduced originally. This lack ofproper control causes, first excessive porosity immediately afterintroduc ing the hydrogen in excessive quantities, and second, scrapcastings owing to the impossibility of conveniently measur ing thehydrogen content when it drops during the casting period.

The present invention provides a method and means of keeping the gascontent of the melt continuously constant without interfering with thecasting operations. In accordance with the invention there is maintainedin the liquid melt a stream of the required gas, this gas current beingcontrolled at a consistent rate of how and discharged into the meltthrough a suitable distributor or diffuser, thereby establishing astationary gas content in the melt. This stationary gas contentrepresents the equilibrium between the amount of gas introduced into theliquid metal from the bubbles risingfrom the distributor to the surfaceof the metal and the loss of gas from the top surface of the melt intothe atmosphere. By altering the flow or the concentration of the gas,any required gas content in the melt can be maintained for any requiredperiod.

The apparatus for introducing the gas is verysimple and convenient tohandle, and does not interfere with the casting operations, especiallyin the case of a bale-out furnace. In the case of a tilting pot, thediffuser can be inserted in the pot some time before the actual castingstarts, and if necessary removed from the pot immedi ately beforecasting, without alteration in the normal foundry operations.

The rate of flow of the gas stream cannot be fixed beforehand Withoutcalibration, as it is dependent not only on the nature of the gas andthe diffuser, but also on the depth of the liquid bath, the nature ofthe alloy being cast, the temperature, and on the condition of the topsurface of the melt. If the depth of thebath increases, the amount ofgas absorbed by the liquidmetal from a given stream of gas through agivendiffuser becomes higher. Further, if the top surface of the melt iskept clean from dross and fiux, the rate of loss of gas into theatmosphere is higher than ifthe top of the melt is left covered withdross or flux, provided the dross or fltii does not react with the gas.It is a matter of a short period of trial to establish with a givenfurnace, alloy, temperature and diffuser, the right rate of flowrequired to provide sound castings from a particular mould.

Once the rate of flow is established in this way it is only necessary tokeep the height of the metal bath and the condition of the metal surfacein the pot consistent in order to maintain the gas content at this levelfor an unlimited time.

In the case of aluminium the gases used are preferably of theoxygen-free type in order to prevent the formation of aluminium oxidewhich, for example, forms if water vapour is used to introduce hydrogen.Compounds useful for the introduction of hydrogen are for examplehydrocarbons such as methane, butane, the mixture of low hydrocarbonssold as butagas, town gas, producer gas. Also partly chlorinatedhydrocarbons, that is trichlorethylene vapour, methyl chloride and so oncan theoretically be used, but have various practical disadvantages. Inthe'case of vapours, it is advantageous to use an inert carrier gas tointroduce these into the liquid metal. The use of pure dry hydrogen assupplied in steel containers for welding and similar purposes has beenfound particularly efiicient. As it is essential that the gas beintroduced under a pressure sufiicient to overcome the hydrostaticpressure of the liquid metal at the bottom of the ladle or crucible, theuse of town gas and producer gas from the normal gas mains is excludedfor most practical cases. These two substances have also thedisadvantage of being saturated with water vapour, which produces acertain amount of dross by reaction with aluminium.

The distributor or diffuser employed in carrying out the invention mayconsist of a straight or curved piece of suitably protected steel pipeor refractory pipe which is equipped with a number of fine holes and isplaced near the bottom of the metal bath.

Two suitable forms of distributor or diffuser are illustrated by way ofexample on the accompanying drawing.

Fig. 1 is an elevation of one form.

Fig. 2 is a section on the plane II-II of Fig. 1.

Fig. 3 is an elevation of the second form.

The diffuser illustrated by Figs. 1 and 2 comprises a steel tube 1 bentas shown and protected by asbestos tape wrapping surrounded byrefractory cement 2. The head of the diffuser has a number of smallholes 3 for the outlet of the gas.

In use the upper end of the diffuser is connected with a supply of therequired gas and, with the gas streaming through the diffuser, it islowered into the metal bath. The rate of flow of the gas can beascertained by any suitable flow meter, for example of the Venturi type.or if no great accuracy is required it may be possible to control therate of flow simply by keeping constant the pressure under which the gasescapes from the diffuser and the head of liquid metal above thediffuser.

The form of diffuser illustrated by Fig. 3 consists simply of a thinvertical steel pipe 1 protected as already described or by refractorymaterial. Gas escapes from the lower end of the tube 1 and bubblesthrough the liquid metal in the crucible A. The pipe 1 is surrounded bya wider vertical tube 2a having ample openings near the bottom andimmediately below the top of the metal bath B. This diffuser has agentle stirring effect in that the rising bubbles draw a stream ofliquid metal through the wider tube 2a.

In certain cases the described means may be supplemented by keeping thelargest part of the metal surface under a lid retaining the hydrogenconcentration at an appropriate level and taking metal out by an openwell of comparatively small surface area.

As an example of the rate of flow necessary for practical purposes, thefollowing case is mentioned: A casting weighing about 8 oz. with a wallthickness of about 14; and ribs on both sides of the main panel was tobe cast in aluminium alloy of the composition Cu 3 Si g 4 Mn 5%,remainder A1 with normal impurities. The

' metal for casting was contained in an oil heated plumbago pot of 250lbs. capacity, the jdepth of the bath being about 15". In order toobtain sound castings it was necessary prior to the present invention totreat the metal with a gassing compound, say ammonium chloride, once inevery 5 minutes. Immediately after treatment the gas content was far inexcess of what was necessary, whilst after 5 minutes it had fallen to anextent so as to raise the scrap percentage to more than 30% of thecastings produced. However, by inserting a diffuser as herein describedand passing through the melt a constant stream of dry pure hydrogen froma steel container at a rate of .5 to 1 litre per minute, the gas contentwas kept permanently just above the minimum limit for the production ofsound castings, whereas in order to produce the same gas content asimmediately after treatment with gassing compound a hydrogen stream ofabout 3 litres per minute was required.

I claim: 1

1. In the production of metal castings wherein shrinkage defects arereduced by having dissolved in the molten metal at the time of casting adefinite content within the limits required to produce a sound castingof a gas which is released unchanged in chemical composition onsolidification of the metal, a method of establishing said content ofdissolved gas comprising bubbling through the metal in a molten stateand in a vessel open to the atmosphere a stream of said gas at apredetermined and constant rate to the point of equilibrium between thegas introduced into the metal and that escaping into the atmosphere fromthe metal, and thereafter maintaining said equilibrium by continuingsaid bubbling at said predetermined and constant rate independently oftime up to the point of casting.

2. In the production of metal castings of aluminum and its alloyswherein shrinkage defects are reduced by having dissolved in the moltenmetal at the time of casting a definite content of hydrogen within thelimits required to produce a sound casting, a method of establishingsaid content of dissolved hydrogen comprising bubbling through the metalin a molten state and in a vessel open to the atmosphere a stream ofhydrogen at a predetermined and constant rate to the point ofequilibrium between the hydrogen introduced into the metal and thatescaping into the atmosphere from the metal, and thereafter maintainingsaid equilibrium by continuing said bubbling at said predetermined andconstant rate independently of time up to the point of casting.

References Cited in the file of this patent UNITED STATES PATENTS 90,158Dormoy May 18, 1869 1,019,965 Kelly Mar. 12, 1912 1,452,364 Davis Apr.17, 1923 1,836,196 Snelling Dec. 15, 1931 1,845,694 Wood Feb. 16, 19322,067,394 Hall Jan. 12, 1937 2,160,812 Alden et al. June 6, 19392,497,119 Fink Feb. 14, 1950 FOREIGN PATENTS 175,462 Germany Feb. 7,1905 684,865 Great Britain Dec. 24, 1952 OTHER REFERENCES Hydrogen andNitrogen As Causes of Gassiness in Ferrous Castings, by Zaplfe and Sims,published in the Transactions American Foundrymens Association (1943vol. 51, pages 517-562. (Pages 519-521 relied on.)

I ournal of The Institute of Metals (British), 1947 vol. 73, pages537-540.

Metal Progress, December 1948, pages 833 and 834.

1. IN THE PRODUCTION OF METAL CASTINGS WHEREIN SHRINKAGE DEFECTS AREREDUCED BY HAVING DISSOLVED IN THE MOLTEN METAL AT THE TIME OF CASTING ADEFINITE CONTENT WITHIN THE LIMITS REQUIRED TO PRODUCE A SOUND CASTINGOF A GAS WHICH IS RELEASED UNCHANGED IN CHEMICAL COMPOSITION ONSOLIDIFICATION OF THE METAL, A METHOD OF ESTABLISHING SAID CONTENT OFDISSOLVED GAS COMPRISING BUBBLING THROUGH THE METAL IN A MOLTEN STATEAND IN A VESSEL OPEN TO THE ATMOSPHERE A STREAM OF SAID GAS AT APREDETERMINED AND CONSTANT RATE TO THE POINT OF EQUILIBRIUM BETWEEN THEGAS INTRODUCED INTO THE METAL AND THAT ESCAPING INTO THE ATMOSPHERE FROMTHE METAL, AND THEREAFTER MAINTAINING SAID EQUILIBRIUM BY CONTINUINGSAID BUBBLING AT SAID PREDETERMINED AND CONSTANT RATE INDEPENDENTLY OFTIME UP TO THE POINT OF CASTING.